PYTHON_INSTDIR is not defined if nanopb_BUILD_GENERATOR=OFF (#764)

Don't install the `_utils` file if the generator is not being installed either.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index cb90012..c9cf43b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -65,10 +65,10 @@
             DESTINATION ${PYTHON_INSTDIR}/proto/
         )
     endforeach()
-endif()
 
-install(FILES generator/proto/_utils.py
-        DESTINATION ${PYTHON_INSTDIR}/proto/)
+    install( FILES generator/proto/_utils.py
+             DESTINATION ${PYTHON_INSTDIR}/proto/ )
+endif()
 
 if(WIN32)
     install(